Just in case this hasn't been posted here yet (didn't find any mention of it through forum search or google search, so, I don't know. Also didn't see it in the master list of games, only saw the EUR version in there)

Codes for MGS Portable Ops USA version
note: I can't take credit for this - found it here: youtube[dot]com/watch?v=7o_W_S7HwoE - it's in the comments, though, not the video. Look at the comment from the youtube user HeartlessSeph, that's where I got this from. Tested it out, and I've completed the game from start to finish, without any problems, using this hack (specifically, the 30 FPS hack - just like other people have reported, the 60 FPS hack breaks the 'sticking to walls' sneaking thing (when you go up to a wall and press your back up against it by pressing triangle)). Made a post about it (and some other things) on the MGS Portable Ops thread as well.